1. Field of the Invention
The invention relates to a process for manufacturing side fold sacks from a flat lying segment of a web of plastic tubular film.
2. Description of the Related Art
Side fold sacks made of plastic, for example polyolefin films, like PE and PP, are manufactured from flat lying segments of a web of plastic tubular film by providing a weld, which runs at right angles and grasps all layers, on one end. Owing to the typically high weight of the fill material and the resulting load on the sacks, said sacks are usually made of especially thick plastic films. On tubular segments made of plastic films of such thickness the bottoms can be affixed only by means of transverse welds, whose execution requires a long welding time. The welding operation requires that the heat be passed through all of the layers of film which, in the area of the side folds, requires heat passage through four layers of film, in order to guarantee the requisite melting and welding together. This thermal conductivity process time, which increases as the thickness of the film and the number of layers increase, results in a significantly long welding period so that the sacks can be manufactured only at low efficiency.
Therefore, the object of the invention is to propose a process of the type described in the introductory part and according to which side fold sacks made of plastic film can be manufactured with greater efficiency.
The invention solves this problem in that one end of the segment of a web of plastic tubular film is provided in such a manner with a staggered cut or a staggered detachment along a perforation that, in a top view of the staggered portion, the rear wall projects beyond the front wall. The upper surfaces of the staggered portion are provided with an application of adhesive as far as into the area of a fold line, which is located adjacent the free bottom edge of the front wall, and the staggered portion is folded along the fold line and over the free edge to be affixed onto the front wall. Such a process for manufacturing side fold sacks made of paper already exists. The invention is based on the surprising knowledge of the benefit of transferring this process to the manufacture of paper sacks, where the bottoms were affixed hitherto by means of welds.
Plastic adhesives, for example polyurethane adhesive or hot melt, are used as the adhesive to cement the folded over staggered portion to the front wall.
The cementing operation can be improved by further subdividing the staggered portion. Expediently, when viewed from above, a portion of the rear wall projects beyond the side folds, and a portion of the side folds project beyond the front wall.
The staggering of the layers may be further increased by forming the side folds in such a manner that the bottom layers of the side folds project beyond the upper layers.
A preferred embodiment provides that the perforation lines are formed in a flat lying web of plastic at intervals equal to the length of the segments of a web of plastic tubular film, before the segments are separated. The side fold tubular web is formed by folding the web sides so as to overlap and simultaneously inserting side folds and affixing a center weld which runs lengthwise. Thus, starting from the part that forms the rear wall, the perforation lines pass in steps over those parts that form the side folds into the part that forms the front wall, whereby the transverse segments of the perforation line are parallel to each other. One embodiment of the invention is explained in detail below with reference to the drawings.